After some suggestion I have made some changes on my code
Guid AgentGUID = Guid.Parse(AgentUID);
var AgentQuery = from a in db.Agents
join u in db.UserLogins on a.AgentUID equals u.UserUID into j
from u in j.DefaultIfEmpty()
where a.AgentUID == AgentGUID
select new { a.AgentUID, a.Name, Login = (u == null ? String.Empty : u.Login), UserUID = ( u == null ? Guid.Empty : u.UserUID), Type = (u == null ? -1 : u.Type) };
foreach (var result in AgentQuery)
{
txtName.Text = result.Name;
txtUsername.Text = result.Name;
if (!string.IsNullOrEmpty(result.Login.ToString()))
{
chkEnableLogin.Checked = true;
txtUsername.Text = result.Login;
txtUsername.ReadOnly = true;
ddlType.SelectedValue = result.Type.ToString();
ViewState.Add("UserUID", result.UserUID);
}
}
The select statement to check Null actually does the trick. Thanks everyone that gives me the hint